U.S. patent number 11,291,875 [Application Number 17/458,887] was granted by the patent office on 2022-04-05 for siloxane and glucoside surfactant formulation for fire-fighting foam applications.
This patent grant is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The grantee listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Ramagopal Ananth, Spencer L. Giles, Katherine Hinnant, Arthur W. Snow.
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United States Patent |
11,291,875 |
Ananth , et al. |
April 5, 2022 |
Siloxane and glucoside surfactant formulation for fire-fighting
foam applications
Abstract
Disclosed is a firefighting composition of the surfactants below
and water. The values of m, n, x, and y are independently selected
positive integers. R is an organic group. R' is a siloxane group.
##STR00001##
Inventors: |
Ananth; Ramagopal (Bryn Mawr,
PA), Snow; Arthur W. (Alexandria, VA), Hinnant;
Katherine (Washington, DC), Giles; Spencer L. (Lorton,
VA) |
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
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Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
(Washington, DC)
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Family
ID: |
1000006220029 |
Appl.
No.: |
17/458,887 |
Filed: |
August 27, 2021 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20210387033 A1 |
Dec 16, 2021 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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16356254 |
Mar 18, 2019 |
11117008 |
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62661824 |
Apr 24, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A62D
1/0071 (20130101); A62C 5/02 (20130101) |
Current International
Class: |
A62D
1/02 (20060101); A62C 5/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Anthony; Joseph D
Attorney, Agent or Firm: US Naval Research Laboratory
Grunkemeyer; Joseph T.
Parent Case Text
This application is a continuation application of U.S. Pat. No.
11,117,008, issued on Sep. 14, 2021, which claims the benefit of
U.S. Provisional Application No. 62/611,824, filed on Apr. 24,
2019. The provisional application and all other publications and
patent documents referred to throughout this nonprovisional
application are incorporated herein by reference.
Claims
What is claimed is:
1. A method comprising: forming a foam from a composition
comprising: a first surfactant having the formula: ##STR00004## a
second surfactant having the formula: ##STR00005## and water;
wherein m and n are independently selected positive integers;
wherein x and y are independently selected non-negative integers;
wherein R is an organic group or H; and wherein R' is a siloxane
group; and applying the foam to a fire.
2. The method of claim 1, wherein the first surfactant has the
formula: ##STR00006##
3. The method of claim 1, wherein m is from 2 to 50.
4. The method of claim 1, wherein n is from 1 to 20.
5. The method of claim 1, wherein x is from 0 to 4.
6. The method of claim 1, wherein y is from 0 to 5.
7. The method of claim 1, wherein R is CH.sub.3-- or H--.
8. The method of claim 1, wherein the composition comprises more
than one of the first surfactants or the second surfactants having
different values of m, n, x, or Y.
9. The method of claim 1, wherein the first surfactant has a
concentration in the composition that is at least the critical
micelle concentration of the first surfactant.
10. The method of claim 1, wherein the first surfactant has a
concentration in the composition of up to 1.0 wt. %.
11. The method of claim 1, wherein the second surfactant has a
concentration in the composition that is at least the critical
micelle concentration of the second surfactant.
12. The method of claim 1, wherein the second surfactant has a
concentration in the composition of up to 1.0 wt. %.
13. The method of claim 1, wherein the composition further
comprises: a solvent having the formula: ##STR00007## wherein p and
z are positive integers.
14. The method of claim 13, wherein p is from 4 to 12.
15. The method of claim 13, wherein z is from 1 to 40.
16. The method of claim 13, wherein the solvent as a concentration
in the composition of up to 1 wt. %.
17. The method of claim 1, further comprising: applying the foam to
a fire in an amount sufficient to extinguish the fire.
Description
TECHNICAL FIELD
The present disclosure is generally related to fire suppressant
materials.
DESCRIPTION OF RELATED ART
Prior to the 1960s, foams based on proteinaceous waste products
were used to extinguish hydrocarbon fuel fires (Ratzer, "History
and Development of Foam as a Fire Extinguishing Medium", Ind. Eng.
Chem. 48, 2013 (1956)). In the 1960s fluorocarbon surfactants were
introduced to fire-fighting foam formulations and largely displaced
the slow acting protein foams (Tuve et al., "Compositions and
Methods for Fire Extinguishment and Prevention of Flammable Vapor
Release", U.S. Pat. No. 3,258,423 (1966); Tuve et al., "A New
Vapor-Securing Agent for Flammable-Liquid Fire Extinguishment",
Naval Research Laboratory Report 6057, DTIC Document No.
ADA07449038, Washington D.C. (1964)). It was proposed that the
fluorocarbon surfactants form an aqueous film under the foam layer
that seals off fuel vapors emerging from the pool surface. The
aqueous film was attributed to spread on the pool surface because
fluorocarbon surfactants reduce the surface tension to an extremely
low value (<17 dynes/cm). The foam layer's role was thought to
protect the aqueous film from heat and was a water delivery
mechanism to the aqueous film. The aqueous film was considered to
be responsible for the high fire suppression performance of aqueous
film forming foam (AFFF). AFFF formulations over time have evolved
into complex recipes with many ingredients to serve multiple
purposes. Many AFFF commercial formulations are understandably
complex and proprietary. Hydrocarbon surfactants were added to the
fluorocarbon surfactants to reach dynamic surface tension more
quickly for spreading of the aqueous film. Other components in
addition to water include: organic solvents (viscosity control,
storage stabilization at subzero or elevated temperatures);
polymers (precipitated barrier formation on polar/alcohol fuels);
salts (surfactant shielding); chelating agents (polyvalent ions
sequestering); buffers; corrosion inhibitors; and biocides (Martin,
"Fire-Fighting Foam Technology," in Foam Engineering: Fundamentals
and Applications; P. Stevenson, Ed.; Ch. 17, Wiley-Blackwell, West
Sussex, UK (2012)). U.S. Pat. No. 5,207,932 discloses some
particularly informative recipe examples. Since their introduction,
they have been used by the civilian and military worldwide
including most airports internationally and are considered the
equivalent of a gold standard in pool firefighting because of their
high fire suppression performance, which is defined more generally
as the ability to extinguish completely a given fire quickly using
minimal amount of solution. The fire performance is defined more
specifically by U.S. MilSpec Mil-F-24385F, which is used to certify
the performance of AFFFs for use in DOD firefighting applications
and probably the most stringent compared to other standards of
performance (e.g., International Civil Aviation Organization-ICAO,
Underwriters Laboratories Inc.-UL) used in civilian applications.
One of the test performed under U.S. MilSpec is a fire extinction
test that specifies that a 6-ft diameter gasoline pool fire be
extinguished in less than 30 s using less than 1 U.S. gallon of
solution.
While fluorocarbon-containing AFFF formulations have been highly
effective, the fluorocarbon surfactants contained in AFFF are found
to pose serious environmental and health hazards (Moody et al.,
"Perfluorinated Surfactants and Environmental Implications of their
Use in Firefighting Foams", Environ. Sci. Tech., 34, 3864 (2000)).
Elimination or replacement of the fluorocarbon surfactant component
in the AFFF formulation is an important and imperative research
objective; legal authority such as U.S. EPA and equivalent European
government agencies have been restricting the use of fluorocarbons
in firefighting foams either on a voluntary basis or by law, and
may in the future require a total discontinuation (Zhang et al.,
"Review of Physical and Chemical Properties of Perfluoro Octanyl
Sulphonate (PFOS) with Respect to its Potential Contamination on
the Environment", Adv. Mater. Res., 518, 2183 (2012)). In addition
to the environmental and health hazards, there has always been an
economic driver in place for many years as the cost of the
fluorocarbon surfactants "represents 40-80% of the cost of the
concentrate" (U.S. Pat. No. 5,207,932).
Fluorine-free surfactant formulations may-significantly reduce the
environmental and health impacts, as they do not contain one of the
most stable bonds between carbon and fluorine in organic chemistry.
However, the problem is that it is extremely difficult to achieve
aqueous film formation without the fluorine due to the inability to
achieve extremely low surface tension (<17 dynes/cm). After
decades of research, the firefighting community has not been able
to find fluorine-free surfactants that reduce the surface tension
to extremely low values. In 2016, a fluorine-free fire suppressing
formulation containing a surfactant composed of a glucoside head
group bonded to a siloxane tail group was custom synthesized (U.S.
Pat. Nos. 9,446,272 and 9,687,686). A formulation containing the
custom synthesized trisiloxane with a glucoside head group, a
hydrocarbon surfactant (Glucopon 215 UP, BASF Inc.), and a solvent
(diglycol butyl ether, DGBE) was able to lower the surface tension
to 20 dynes/cm to achieve the aqueous film formation marginally on
a limited number of fuels (kerosene and jet fuel) having relatively
high surface tension. The siloxane formulation was unable to form
an aqueous film on n-heptane or gasoline fuel, which is employed in
U.S. MilSpec tests (Mil-F-24385F). Furthermore, the siloxane
surfactant was prepared by a multistep synthesis with relatively
low yield, which is of questionable practicality for large scale
synthesis. Blunk et al. also considered four, non-glucoside,
trisiloxane surfactants as counter-examples for comparison that did
not form the aqueous film. They were tri-siloxanes with oxyethylene
head group (4, 6, and 12 unit lengths) terminated with hydroxyl
similar to the commercial tri-siloxane surfactant component
described herein. However, Blunk et al. rejected the trisiloxanes
with oxyethylene head group for fire suppression on the basis that
the siloxanes did not form the aqueous film. In summary, no
fluorine-free replacement surfactants have been found with film
formation ability comparable to that of AFFF on low surface tension
fuels (gasoline and heptane).
To compensate for the loss of the aqueous film, the foam industry
(e.g., RF6, Solberg, Inc. product and Angus 3%, National Foam, Inc.
product) developed fluorine-free foams that reduce drainage and
hold more water in the foam layer. The increased liquid content in
the foams was achieved by using hydrocarbon surfactants and
viscosity modifying additives to control liquid loss by drainage
from the foams. However, these approaches to replacing the
fluorocarbon surfactants sacrifice AFFF's high fire suppression
performance because of the use of less fuel resistant hydrocarbon
surfactants and excess solution for comparable fire extinction
time. Because only a limited amount of the solution can be carried
to the fire site, the commercial fluorine-free foams will not be
able to put out large fires as quickly as AFFF on a per unit mass
of liquid basis. As a result, the fluorine-free formulations are
not expected or claimed to have passed the more stringent U.S.
MilSpec (Mil-F-24385F) by the manufacturers. However, some of the
commercial fluorine-free foams have been qualified by European
standards (ICAO) for civilian firefighting applications.
In summary, all surfactant AFFF formulations to date that meet the
Military Specification (MilSpec) requirements for fire
extinguishing (Mil-F-24385F) contain fluorocarbon surfactants.
Fluorine-free firefighting foam formulations do exist but to date
have not met the MilSpec requirements.
BRIEF SUMMARY
Disclosed herein is a composition comprising a first surfactant
having the formula (1), a second surfactant having the formula (2),
and water. The values of m, n, x, and y are independently selected
positive integers. R is an organic group. R' is a siloxane
group.
##STR00002##
Also disclosed herein is a method comprising: forming a composition
of the first surfactant, the second surfactant, and water.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation will be readily obtained by reference
to the following Description of the Example Embodiments and the
accompanying drawings.
FIG. 1 shows fire performance extinction time versus liquid
surfactant formulation flow rate to foam generating apparatus onto
a 19-cm diameter heptane pool fire, with a 60 sec preburn and 1 cm
lip. U.S. MilSpec qualified fluorocarbon surfactant containing
commercial (Buckeye Fire Equipment Co. 3% MIL SPEC AFFF (BFC-3MS))
AFFF (open circles), RefAFFF (solid circles), siloxane
(fluorine-free) surfactant formulation with composition shown in
Table 1 (square), and the two leading commercial fluorine-free
formulations, RF6 (X) and Angus 3% (triangle). (Data points on
x-axis represent no extinction). Commercial fluorine-free
concentrate viscosities are 20 to 1500 times the MIL-F-24385F
specification and the present invention is within the MilSpec (see
Table 3).
FIG. 2 shows fire performance foam spread time to cover the pool
surface versus liquid surfactant formulation flow rate during fire
extinction, for the same conditions as FIG. 1. Below 100 mL/min
liquid flow, RF6 spreads and covers the pool quickly but takes 60 s
or more to extinguish the fire unlike the siloxane formulation. A
liquid flow of 100 mL/min corresponds to 3.5 kg/m.sup.2/min
application rate and a foam flow rate of 840 mL/min.
FIG. 3 shows 19-cm diameter heptane pool fire performances of
extinction time vs liquid surfactant solution flow rate for the
siloxane formulation of two surfactants (square), and the
individual surfactants alone (circle and triangle) showing
synergistic extinction. A liquid flow of 100 mL/min corresponds to
3.5 kg/m.sup.2/min application rate and foam flow rate of 840
mL/min. (Data points on x-axis represent no extinction).
FIG. 4 shows the change in foam layer thickness due to fuel induced
degradation vs time for the siloxane formulation of two surfactants
(square), and the individual surfactants alone (circle and
triangle) showing synergistic foam stability. Foams are applied
from the foam generation device on to a hot heptane pool placed in
a beaker in the absence of fire until a 4-cm thick layer builds.
The bottom part of the beaker containing heptane pool was place in
a hot water bath to maintain constant temperature 60.degree. C.
FIG. 5 shows the use of analytical .sup.1H NMR data from
synthesized poly(oxyethylene) trisiloxane (m=0, m=1, m=2)
surfactants for estimation of structural descriptors in analog
commercial surfactants (Dow Corning: 502W, 501W and 67A; Momentive:
Silwet L-77; Gelest: SIH6185 m=6-9).
FIG. 6 shows the effect of number of oxyethylene units (displayed
in FIG. 5) on comparative 19-cm diameter heptane pool fire
suppression performance of commercial poly(oxyethylene) trisiloxane
surfactants (502W, 501W, L-77, Gelest 6-9, and 67A), where the
commercial trisiloxane surfactant is used to replace the
fluorosurfactant in RefAFFF formulation shown in Table 1. A liquid
flow of 100 mL/min corresponds to 3.5 kg/m.sup.2/min application
rate and foam flow rate of 840 mL/min. (Data points on x-axis
represent no extinction).
FIG. 7 shows the effect of a hydrocarbon surfactant's head and tail
sizes on 19-cm diameter heptane pool fire extinction performance
for the siloxane formulations with Glucopon225DK having x=0.7,
n=8-10 (square), with Glucopon215UP having x=0.5, n=8-10
(triangle), with Glucopon600UP having x=0.4, n=12-14 (diamond),
with TritonCG425 having x=unknown, n=8-14 (star) compared with AFFF
(circle). Larger values of x and n represent larger head and tail
sizes of the alkyl poly(glycoside) surfactant structure shown in
FIG. 1. A liquid flow of 100 mL/min corresponds to 3.5
kg/m.sup.2/min application rate and a foam flow rate of 840 mL/min.
(Data points on x-axis represent no extinction).
FIG. 8 shows the effect of the solvent's oxyethylene length on
19-cm diameter heptane pool fire performance for the siloxane
formulation with diethylene (squares) and triethylene (circles)
glycol monobutylethers. (Data points on x-axis represent no
extinction).
FIG. 9 shows the effects of varying the ratio of 502W/Glucopon225DK
(1/3, 2/3, 3/2) surfactants on the 19-cm diameter heptane pool fire
performance of the siloxane formulation, while keeping the total
surfactant constant. 83 mL/min liquid flow corresponds to 2.9
kg/m.sup.2/min liquid flux (used in 28 ft.sup.2 pool MIL-F-24385F)
and a foam flow of 698 mL/min. (Data points on x-axis represent no
extinction).
FIG. 10 shows the effects of varying total surfactant from 0.125%
to 0.5% in the siloxane formulation on 19-cm diameter heptane pool
fire suppression performance, while keeping the total siloxane
surfactant to hydrocarbon surfactant ratio constant (3:2). (Data
points on x-axis represent no extinction).
FIG. 11 shows example siloxane surfactants. Top row: all have
identical trisiloxane tails with unspecified distribution of
oxy-ethylene units (n) in the head group terminated with a hydroxyl
unit. However, FIG. 5 gives estimated values of n=15, 13.5, 12.5,
10.5, and 10 for 502W, 501W, L77, GelestSiH6185, and 67A
respectively based on analytical NMR data. GelestSiH6185 has n=6 to
9 and 502W has larger head than 501W and 67A. The other three are
similar but have head groups terminated with a methyl unit. Bottom
row: grafted surfactants with multiple tails and heads, but
differing in the number of siloxane, oxy-ethylene and oxy-propylene
units. Silphos is similar but has an anionic head group.
FIG. 12 shows hydrocarbon surfactants. Top row: SDS and Alpha
Foamer differ by an oxy-ethylene unit in the head group; Alpha
Foamer has a distribution of chain lengths including the dodecyl
similar to SDS's tail. Glucopon 215UP has x=0.5, n=8-10,
Glucopon225DK has x=0.7, n=8-10, and Glucopon600UP has x=0.4,
n=12-14. Bottom row: Tergitols have twin hydrocarbon tails and
similar head groups containing different length poly oxy-ethylene
units. Tergitol TMN6 also has pendant methyl units. Triton is
linear with phenyl linker.
FIG. 13 shows decrease in foam thickness with time when a 4-cm
layer is placed on top of hot 60.degree. C. heptane pool.
Comparison with commercial fluorine-free foams and reference AFFF.
The bubbles close to the pool surface coalesce and drain liquid
more rapidly than the bubbles farther from the interface; very
little change in bubbles occurs when the foam layer is placed on
hot water. Foam degradation is induced by the fuel.
FIG. 14 shows percent suppression in heptane vapor concentration
versus time when a 4-cm thick foam layer is applied onto a hot
60.degree. C. heptane pool at time zero. As time progresses,
heptane vapor permeates through the foam layer, which also
decreases in its thickness as shown in FIG. 13. The time (indicated
by vertical arrows) when the fuel concentration above the foam
surface reaches lower flammability limit indicates the degree of
fuel resistance of the surfactant formulation. Large variation is
seen among the three trials for the commercial foam Angus 3%.
FIG. 15 shows fuel permeation rate and degradation of foam by the
fuel vapor relative to AFFF, which is shown near origin. The
fluorine-free siloxane foam (Siloxane A-Form) and RF6 (the leading
commercial fluorine-free foam) are closest to the origin compared
to the rest of the commercial surfactants tested. But, the siloxane
foam suppresses the fuel vapor permeation and foam degradation by
using significantly less solution than the commercial RF6. (Solid
markers: surfactant solutions, open markers: custom formulations,
"Form", line: best-fit.)
FIG. 16 shows 12-cm heptane pool fire extinction versus fuel
permeation rate relative to AFFF showing a direct correlation. Fire
extinction is within a factor of 3 of AFFF for the fluorine-free
foams of Siloxane A-Form and RF6 (Solberg Inc.) but use
significantly different amounts of solution to do so. Other
commercial surfactants are far inferior as indicated by the
distance away from the origin, where AFFF is placed.
FIG. 17 shows dynamic surface tension versus time for the siiloxane
formulation and other hydrocarbon and fluorocarbon surfactant
formulations. At small times, the siloxane formulation exhibits
unique and rapid decrease in surface tension relative to the other
commercial formulations shown. At long times, the siloxane
formulation has slightly greater surface tension than AFFF.
FIG. 18 shows bench-scale (19-cm dia.) and large scale (6-ft
dia.MIL-F-24385F) extinction of heptane pool fire showing
extinction times of Siloxane-Gluc225 surfactant formulation in
Table 1 relative to the RefAFFF and commercial formulation (Buckeye
3%) at different measured foam application rates. For the 6-ft
fire, the foam and liquid flux values correspond to 2 and 3
gallons/min solution flow rates.
FIG. 19 shows bench-scale (19-cm dia.) and large scale (6-ft dia.
MIL-F-24385F) extinction of heptane pool fire showing extinction
times of Siloxane-Gluc225 surfactant formulation in Table 1
relative to the RefAFFF and commercial formulation (Buckeye 3%) at
different liquid application rates, which are obtained by dividing
the foam application rates with measured foam expansion ratios.
FIG. 20 shows bench-scale (19-cm dia.) extinction of heptane pool
fire showing synergisms in extinction time of Siloxane-Gluc
mixtures listed in Table 4 relative to the solutions of individual
components, which also contain 0.5% DGBE. The data points shown
along the x-axis represent no extinction data after 180 s of foam
application.
FIG. 21 shows synergisms in measured foam degradation rates for a
4-cm thick (initial thickness) foam layer covering heptane pool at
60.degree. C. with time for the Siloxane-Gluc225, Siloxane-Gluc600,
and Siloxane-Gluc215 listed in Table 4, and for the individual
surfactant components (0.5% Gluc215, 0.5% Gluc225, 0.5% Gluc600,
and 0.1% 502W. All four solutions contain 0.5% DGBE.). Error bars
represent one standard deviation calculated between three
trials.
FIG. 22 shows measured heptane vapor permeation rates for a 4-cm
thick (initial thickness) foam layer covering heptane pool at
60.degree. C. with time for the Siloxane-Gluc225, Siloxane-Gluc600,
and Siloxane-Gluc215 listed in Table 4, and for the individual
surfactant components (0.5% Gluc215, 0.5% Gluc225, 0.5% Gluc600,
and 0.1% 502W. All four solutions contain 0.5% DGBE. Error bars
represent one standard deviation calculated between three
trials.
FIG. 23 shows a comparison of foam spread time to fully cover the
pool surface during the heptane pool fire suppression with the fire
extinction time at different foam application rates for the 19-cm
dia. bench-scale heptane pool. Foam is delivered at the center of
the pool at a constant flow rate and allowed to spread. Spread
times (open symbols) and extinction times (closed symbols) are
shown for Siloxane-Gluc225 and RefAFFF listed in Table 4. Data
points on x-axis (y=0) show flow rates where fire was not
extinguished in 180 s.
FIG. 24 shows dynamic surface tension versus bubble's age for the
Siloxane-Gluc225 and RefAFFF formulations listed in Table 4 at
25.degree. C., and a commercial AFFF (Buckeye 3%) formulation. Also
shown are the surface tensions of individual surfactants (0.5% 502W
with 0.5% DGBE and 0.5% Glucopon 225 DK with 0.5% DGBE solutions)
for comparison with the surfactant mixture and show lack of
synergisms.
FIG. 25 shows static surface tension at different volume % of the
total surfactant concentrate (sum of 502W and Gulcopon 225 DK or
215 CS UP or 600 CS UP concentrates or sum of Capstone.TM. 1157 and
Glucopon 215 CS UP concentrates supplied by the manufacturers, see
Table 4) to determine CMC at 20.degree. C.
FIG. 26 shows initial bubble size distribution for RefAFFF and
Siloxane-Gluc225 formulations listed in Table 4, 2 minutes after
large-scale foam generation at 2 and 3 gpm foam solution flow rates
through an air-aspirated MIL-F-24385 nozzle.
FIG. 27 shows initial bubble size distribution for RefAFFF and
Siloxane-Gluc225 formulations listed in Table 4, 30 seconds after
bench-scale foam generation with a sparger at 1000 mL/min foam flow
rate and fed into the DFA cylinder.
FIG. 28 shows bubble coarsening as indicated by the average bubble
sizes calculated from bubble size distributions measured at
different times after foam generation for RefAFFF and surfactants
listed in Table 4 for large scale foams generated using
air-aspirated MIL-F-24385 nozzle.
FIG. 29 shows bubble coarsening as indicated by the average bubble
sizes calculated from bubble size distributions measured at
different times after foam generation for RefAFFF and surfactants
listed in Table 4 for bench-scale foams generated using a sparger
at 1000 mL/min foam flow rate.
FIG. 30 shows amount of liquid drained from the bottom of a foam
column with time for RefAFFF and Siloxane-Gluc225 formulations
listed in Table 4 for large-scale foam generation using
air-aspirated MIL-F-24385 nozzle.
FIG. 31 shows amount of liquid drained from the bottom of a foam
column with time for RefAFFF and Siloxane-Gluc225 formulations
listed in Table 4 for bench-scale foam generation by using a
sparger at 1000 mL/min foam flow rate.
FIG. 32 shows expansion ratio versus foam flow rate for RefAFFF,
Siloxane-Gluc225, Siloxane-Gluc215, and Siloxane-Gluc600
formulations listed in Table 4 for the bench scale extinction
apparatus sparger generation method at 1000 mL/min foam flow
rate.
FIG. 33 shows a thermal stability test of 3% siloxane concentrate
prepared with 6.66% 502W, 10% Glucopon225DK, 16.66% DGBE in
distilled water and aging the concentrate at 65.degree. C. for 10
days in an oven as per MIL-F-24385F. 19-cm heptane pool fire
extinction was conducted before (solid square) and after aging
(open square) to show no loss in fire extinction performance.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
In the following description, for purposes of explanation and not
limitation, specific details are set forth in order to provide a
thorough understanding of the present disclosure. However, it will
be apparent to one skilled in the art that the present subject
matter may be practiced in other embodiments that depart from these
specific details. In other instances, detailed descriptions of
well-known methods and devices are omitted so as to not obscure the
present disclosure with unnecessary detail.
Described is a preparation of fluorine-free surfactant formulations
to generate foams that have high fuel vapor resistance property per
unit volume of solution comparable to that of the firefighting foam
used currently, world-wide, Aqueous Film Forming Foam (AFFF), which
contains fluorocarbon surfactants with significant environmental
impact. It is demonstrated that the fuel vapor resistance property
leads to extinguishment of hydrocarbon pool fires by blocking fuel
supply to the fire with an efficiency approaching that of AFFF even
though the formulation may not have extremely low surface tension,
and may not form the aqueous film. As an example, a surfactant
formulation composed of trisiloxane poly(oxyethylene) and alkyl
polyglucoside surfactants and other components is shown to spread
quickly, suppress the fuel vapors, and extinguish a pool fire using
smaller amount of solution compared to the leading commercial
fluorine-free foams, and closer to the values measured for AFFF.
Described are surfactant structural features, formulation
compositions' effect on the foam's resistance to the fuel vapors
emerging from the pool surface that correlate with fire suppression
effectiveness, and dynamic surface tension that can affect
foamability. The structural features include a range of head and
tail dimensions. Compositions include the range of relative amounts
of siloxane to hydrocarbon surfactants to achieve synergistic
extinction and increased foam spreading on the pool surface. Fuel
vapor resistance is quantified by the ranges of fuel/heat induced
foam degradation and fuel vapor permeation rate relative to AFFF.
Dynamic surface tension shows time scale for lowering the surface
tension of a freshly formed bubble and foamability is indicated by
the expansion ratio (or liquid content).
It has been demonstrated that the fuel vapor resistance property of
surfactants is crucial for fire suppression efficiency rather than
a liquid layer either in the form of aqueous film formation or high
liquid content of foams ("Measuring Fuel Transport through
Fluorocarbon and Fluorine-free Firefighting Foams", Fire Safety
Journal, 91, 653-661 (2017) and "Influence of Fuel on Foam
Degradation for Fluorinated and Fluorine-free Foams", Colloids and
Surfaces A, 522, 1-17 (2017)). The disclosed formulation does not
form the aqueous film on n-heptane fuel but forms a foam layer,
which is effective in suppressing fuel vapors emerging from the
pool from reaching into the fire. The amount of surfactant solution
contained in the foam used for suppressing a fixed size fire in
fixed time is less than the leading commercial fluorine-free foams
available to date and is 50% more than that used by AFFF. As a
result, the formulation has fire suppression effectiveness well
above the existing fluorine-free formulations and is more than 50%
fire suppression effectiveness of AFFF's based on benchtop
measurements. The superior fire suppression effectiveness is due to
increased oleophobicity of the trisiloxane tail that blocks the
fuel vapor permeation through foam covering the pool surface while
maintaining amphiphilicity with increased oxyethylene head group
size to reduce fuel/heat induced foam degradation. Also significant
is the synergistic interaction with hydrocarbon co-surfactant,
where the fuel/heat induced foam degradation and fire extinction
times are smaller for the combination of the surfactants compared
to those for the two surfactants individually. The synergism
reduces foam degradation by heptane and blocks the fuel permeation
and contributes to faster extinction without using excess solution.
The polar head group of the hydrocarbon surfactant can also
significantly enhance the synergism.
The disclosed composition is a formulation that includes two
classes of surfactants: poly(oxyethylene)-trisiloxane surfactants
and poly(glucoside)-alkane surfactants. General chemical formulas
for these two surfactants are shown in formulas (1) and (2). The
general structures of these two surfactant classes may be available
as commercialized surfactants and analytical or custom synthesized
surfactants. The parameters m, n, and x are all positive integers,
and y is a non-negative integer. For example, m may be between 2 to
50, y may be between 0 and 5, n may be between 1 and 20, and x may
be between 0 and 4. The C.sub.nH.sub.2n+1 group may be linear or
branched. R can be any functional group including --OH and
--CH.sub.3--. R' can be any siloxane group such as
--Si(CH.sub.3)[OSi(CH.sub.3).sub.3].sub.2 or
--Si--[O--Si(CH.sub.3).sub.2].sub.q--O--Si(CH.sub.3).sub.3, where q
is a positive integer such as 2. It is demonstrated that when a
member of each class is combined in a foam generating formulation,
the foam produced displays an effective fire suppression
capability, depending on the values of the parameters. It may or
may not also include a solvent whose general class of structure is
depicted in Eq. (3) with parameters p and z being positive
integers. For example, p may be between 4 and 12, and z may be
between 1 and 40. Formulations were prepared by mixing the three
components in proportions shown in Table 1.
##STR00003##
TABLE-US-00001 TABLE 1 Fluorine-free formulation containing a
siloxane surfactant (e.g., 502W, Dow Corning Inc.), a hydrocarbon
surfactant (e.g., Glucopon225DK, BASF Inc.), a solvent
(diethyleneglycol butylether, DGBE, Dow Chemical Co.) in distilled
water. Also shown is a reference AFFF formulation (RefAFFF)
containing a fluorocarbon surfactant (Capstone 1157, Chemours
Inc.). Siloxane Formulation RefAFFF Formulation.sup.1 0.2% siloxane
surfactant (502W) 0.2% Capstone 1157 0.3% hydrocarbon surfactant
0.3% Glucopon215 CS UP (Glucopon225DK) 0.5% solvent (DGBE) 0.5%
DGBE 99% distilled water 99% distilled water .sup.1It has been
shown that RefAFFF passed the 28 ft.sup.2 U.S Mil-F-24385F fire
test with an extinction time of 26 s, burnback time of 562 s, 25%
liquid drainage time of 317 s, foam expansion ratio of 7.5 (Hinnant
et al., "An Analytically Defined Fire-Suppressing Foam Formulation
for Evaluation of Fluorosurfactant Replacement" J. Surfactants and
Detergents, 21(5), 711-722 (2018))
The components of the RefAFFF formulation are: Glucopon.RTM. 215 CS
UP (an alkyl polyglucoside concentrate contributed by BASF
Corporation, Ludwigshafen, Germany and referred to as "Gluc215"
(Hinnant et al., Surfactant and Detergents, 21, 711-722, (2018))
(For 215UP, x is 0.5 and n is 8 to 10. For 225DK, x is 0.7 and n is
8 to 10. For 600UP, x is 0.4 and n is 12 to 14); Capstone.TM. 1157
(fluorotelomer sulfonamide alkylbetaine concentrate contributed by
Chemours Inc., Wilmington, Del. and referred to as "Cap") (Hinnant
et al. (2018)); Butyl Carbitol.TM. (Dow Chemical Co., Midland,
Mich. purchased as diethyleneglycol butylether, "DGBE", from Sigma
Aldrich, St. Louis, Mo.) (Hinnant et al.). The RefAFFF composition
and properties have been previously characterized in Hinnant et al.
(2018).
The Siloxane-Gluc215 formulation was prepared by replacing Cap in
RefAFFF with Dow Corning.RTM. 502W Additive, which is a silicone
polyether copolymer, a 100% by weight concentrate contributed by
Dow Corning Co., Midland, Mich. density 0.97 g/cm.sup.3. The
Siloxane-Gluc225 and Siloxane-Gluc600 formulations were prepared by
replacing the BASF Glucopon.RTM. 215 CS UP with Glucopon.RTM. 225DK
(an alkyl polyglucoside, a 68-72% by weight concentrate in water,
contributed by BASF Corporation and referred to as "Gluc225" in
this paper, density 1.13 g/cm.sup.3) and with Glucopon.RTM. 600 CS
UP (50 to 53% by weight concentrate) in the Siloxnae-Gluc215
formulation respectively. The resulting solutions were used for
generating foams for fire suppression as well as for foam and
solution properties' measurements.
U.S. MilSpec compliant commercial AFFF formulations are typically
sold as 3% or 6% concentrates, such that the final formulation used
for generating the foam should contain 3% or 6% of the concentrates
in water respectively. Buckeye Fire Equipment Company, Kings
Mountain, N.C. (BFC-3MS, Lot #120050, 2003) and Dafo Fomtec AB,
Tyreso, Sweden (FOMTEC AFFF 3% M USA, Batch #US-16-07-07, Aug. 4,
2016) provided 3% concentrates. They were used as received for the
analytical characterization described by Hinnant et al. The Buckeye
and Fomtec concentrates were diluted with water at 3% by volume for
generating the foams for fire suppression.
Dynamic surface tension was measured using a bubble pressure
tensiometer (Model BP2, KRUSS, Hamburg, Germany) as a function of
bubble's age (1/frequency, 10 ms to 10000 ms). The tensiometer
generates bubbles at a capillary tube lip (0.22 mm diameter)
continuously at a specified frequency by pushing nitrogen through
the capillary immersed in a surfactant solution. Surfactant
diffuses from the solution to the bubble surface, where it gets
absorbed and suppresses the surface tension. Pressure inside the
bubble increases and reaches a maximum when the bubble diameter is
equal to the capillary tube diameter before the bubble detaches
from the capillary. Surface tension is calculated from the measured
maximum pressure using Young's equation. Critical micelle
concentrations (CMC) and static surface tensions for the
Siloxane-Gluc600, Siloxane-Gluc215 and Siloxane-Gluc225 were
measured using a ring (radius 9.58 mm, wire radius 0.185 mm)
tensiometer at 20.degree. C. (Du Nouy Model Sigma 701, Biolin
Scientific Inc., Gothenburg, Sweden). Surface tension was measured
at different concentrations of the total surfactant. CMC values
were determined from the log plot of surface tension against volume
% of the sum of 502W and Glucopon surfactant concentrates supplied
by the manufacturers. Interfacial tensions were measured with the
ring tensiometer between n-heptane and the siloxane formulations at
20.degree. C. The viscosity was measured at 20.degree. C. using a
Cannon.TM.-Fenske viscometer (Fisher Model 50 13616B, capillary
size #50).
Foams can be generated using a device that mixes air and water at
different ratios known as the expansion ratio (e.g., volume of
foam/volume of liquid). As an example, foams are generated by
sparging air continuously at a constant rate through a porous disc
while feeding solution continuously to maintain a constant liquid
column height (3-cm) above the porous disc (25-50 m pores, 1.9-cm
diameter) by using a leveling system. Foam collects to form 5.5-cm
thick layer above the solution surface while flowing out from a
2.5-cm diameter outlet tube connected to the cap of a 0.7-liter
plastic bottle (7.6-cm diameter, 15.9-cm height). Foam flow rate is
maintained constant during fire extinction and are measured by
recording time taken to collect 500 mL volume before and after fire
extinction. Foam expansion ratio (volume of foam/weight of foam) is
also measured before and after each fire extinction experiment in
order to calculate liquid flow rate (foam flow rate/expansion
ratio). To apply the foam continuously on to burning fuel pool, the
outlet tube from the foam generating plastic bottle is placed about
1-inch above the pool surface. The foam is applied directly to the
center of a burning heptane pool (circular shape) and allowed to
spread to the edges until fire extinction or a maximum time of 3
minutes. Extinction experiments are conducted at different values
of liquid (or foam) flow rates. The heptane pool is allowed to burn
for 60 s (preburn time) prior to the foam application. The pool
consisted of 1-cm thick fuel layer above a 5-cm thick water layer.
The fuel level is maintained at 1-cm below the rim of the 19-cm
diameter crystallizing dish to accommodate the foam and prevent
overflow of the fuel by using a leveling system. The apparatus used
for generating the foams and conducting fire extinction were
developed previously (Hinnant et al., "An Analytically Defined
Fire-Suppressing Foam Formulation for Evaluation of
Fluorosurfactant Replacement" J. Surfactants and Detergents, 21(5),
711-722 (2018)).
The foams were characterized by measurements of initial bubble
size, initial expansion ratio, and liquid drainage rate versus time
at bench scale and large scale. Expansion ratio is the volume of
foam per unit volume of liquid contained in the foam. Expansion
ratio was measured by generating a fixed volume of foam into a
graduated cylinder and measuring the foam's mass, which was
converted to liquid volume using the density of water. Foams were
generated with air externally using the extinction apparatus at a
constant foam flow rate between 950 to 1000 mL/min and fed directly
to fill the glass container of a Dynamic Foam Analyzer (DFA100,
KRUSS GmbH, Matthews, N.C.) for the bench-scale measurements. The
DFA container (40 mm diameter, 25 cm height cylinder) has part of
its walls (inner and outer) shaped flat. The flat surface is in
contact with the bubbles of the foam. A prism attached to the flat
surface reflects light forming a mirror image of the foam-surface
bubbles at a video camera's focal plane. The camera is placed 13 cm
from the top of the foam column. Starting within one minute of the
foam generation, the video images are continuously analyzed by the
computer software (ADVANCE) to provide plots of bubble size
distributions, average bubble size, and the position of
foam-solution (drained solution) interface with time. In addition
to the bubble size distributions, the plots provide bubble
coarsening and liquid drainage rates from the 25 cm height foam
column.
As prescribed in MilSpec MIL-F24385F, the foam is sprayed on to an
aluminum plate and the foam is collected into a container for
characterization. The foam fills a rectangular glass container (4.2
cm.times.4.2 cm.times.30.5 cm) affixed with a millimeter ruler
positioned in front of a digital camera (Nikon DSLR) placed at
13-cm height of the 30.5-cm foam column. Images of the foam in the
column with the ruler were taken within two minutes of the foam
being collected. The diameter of 50 to 100 bubbles for three
independent images (150 to 300 total bubbles) were measured using
open source software (ImageJ). The liquid drainage rate was
measured by collecting a 28-cm height column of foam into a 500 mL
graduated glass cylinder (5-cm diameter) and measuring the change
in liquid level at the base of the container with respect to
time.
Foam degradation was measured following a procedure similar to
those described elsewhere (Hinnant et al., "Influence of Fuel on
Foam Degradation for Fluorinated and wo Fluorine-free Foams",
Colloids and Surfaces A, 522, 1-17 (2017)). The foam height was
measured as a function of time in a 100 mL glass beaker (5.0 cm
diameter) in a water bath (150 mL) controlled by using a heating
tape and a thermostat set at 60.degree. C., based on previous
measurements of the foam-pool interface temperature during fire
extinction (Conroy et al., "Surface Cooling of a Pool fire by
Aqueous Foams", Combustion Science and Technology, 189, 806-840
(2017)). The preheated liquid fuel (55 mL) was then poured into the
beaker using a funnel, leaving a head space of 4-cm height to
accommodate the foam layer. Foam was generated using nitrogen gas
at a constant foam flow rate between 950 to 1000 mL/min using a
constant nitrogen flow of 900 mL/min by the sparging method and fed
directly into the beaker. A spatula was used to scrape excess foam
from the top of the beaker, forming an even 4 cm foam layer on top
of the preheated liquid fuel. Care was taken to keep the water bath
level just below the foam-fuel interface in the beaker so that the
foam was not heated by the water bath directly. A video camera
monitored the foam height over time. The thickness of foam was
determined by measuring the height of the top surface of the foam
layer and the liquid fuel surface seen in the recorded video. In
the cases where a gas bubble or "gap" lifted the entire foam layer
from the liquid fuel surface, the volume of the gap was excluded
from the total foam height. The "gap" is a result of foam bubbles
bursting and coalescing to form a single bubble that spans the
width of the container when in contact with the liquid fuel
(Hinnant et al.). Thus, the gap contains the nitrogen that was
inside the foam bubbles and also contains the warm fuel vapor.
A flux chamber was used to measure fuel flux through a foam layer
with an initial thickness of 4 cm, placed on a hot heptane pool. A
two-piece transport chamber was designed to quantify the initial
dynamics of fuel transport as soon as a foam layer was placed on
the pool. Similar experiments were conducted at room temperature
using a plastic chamber previously (Hinnant et al., "Measuring Fuel
Transport through Fluorocarbon and Fluorine-free Firefighting
Foams", Fire Safety Journal, 91, 653-661 (2017)). The chamber was
modified to conduct measurements on a heated fuel. The chamber
consisted of a bottom glass cylindrical piece, 5 cm in diameter, 8
cm long and a top glass cylindrical piece. The pieces were joined
together by placing an O-ring in an extruded glass section of the
bottom piece and matching the extruded glass section of the top
piece. A large black clamp was then screwed tightly to put pressure
on the O-ring and seal the container. The top piece transitioned
from a cylinder, 5 cm in diameter, into a cone shape with the top
containing a screw cap that affixed a porous glass frit to the
inside of the top piece. The glass frit, pore size 25-50 .mu.m, was
3 cm in diameter, and positioned 1 cm from the open end of the top
piece. The screw cap on the top piece had an additional outlet with
1/4'' plastic tubing that extended to a Midac FTIR (Fourier
Transform Infrared Spectrometer, Midac I Series, Model 14001,
Serial 587, Midac Corporation, Westfield, Mass., USA). The sparger
brought nitrogen into the transport chamber to sweep fuel vapors
from the foam surface. The outlet then carried this swept gas to an
FTIR. The bottom glass piece was filled with 70 mL of n-heptane,
leaving 4 cm of headspace in the bottom piece. The piece was then
lowered into a water bath, heated by an external thermostat heating
tape, and the n-heptane was heated to 60.degree. C. Foam was then
generated using a sparger method with nitrogen (25-50 .mu.m pore
size, at a constant foam application rate between 950 to 1000
mL/min using a constant nitrogen flow rate of 900 mL/min) directly
into the bottom piece. A spatula was used to scrape foam from the
bottom piece, forming a flat level surface of the foam layer
covering the entire pool surface. The O-ring was then put in place
and the system was closed tight. Nitrogen flowed from the sparger
into the top piece at a rate of 500 mL/min. The inlet to the FTTR
was then opened and the system began to take measurements of fuel
concentration as ppm versus time. A nitrogen bypass on the FTIR
allowed us to analyze large n-heptane quantities over a longer
period of time without saturating the instrument. The nitrogen
bypass flow rate was 100 mL/min. The test was stopped when the
n-heptane surface was exposed as the foam layer degrades over time
and the FTIR signal reached a steady value of 6000 ppm at
59.degree. C. (corresponds to a fuel flux 1.4.times.10.sup.4
mol/cm.sup.2/s) or 2480 ppm at 18.degree. C. Nitrogen flow rates
were controlled using Sierra Instrument flow controllers (Sierra
Instruments, Monterey, Calif., USA, two 840-L-2-OV1-SV1-D-V1-S1
controllers with flow ranges 0-1000 sccm for foam generation and
0-2000 sccm for nitrogen sweep, one 840-L-2-D-S1 controller with
flow range 0-500 sccm for nitrogen bypass). Tests were run in
triplicate. The measured concentration of fuel by FTIR was
converted to molar flux by multiplying the heptane vapor
concentration (volume fraction, # ppm/1000000) with molar flow rate
(4.45.times.10.sup.4 mol/sec) of total nitrogen flowing (600
mL/min) through the FTIR and dividing by the surface area of the
foam layer (19.63 cm.sup.2).
Fire extinction can be conducted by applying the foams from the
foam generating device on to a burning liquid fuel pool at
different application rates. For example, fire extinction testing
has been conducted on benchtop 19-cm heptane pool fires with 60
second preburn, and 1-cm lip to accommodate a foam layer on top of
the pool. Examples of such testing results are depicted in FIG. 1
where the extinction time is measured as a function of measured
solution/liquid flow rate for a 2:3:5 formulation ratio of
poly(oxyethylene)-trisiloxane:poly(glucoside)-alkane:diethyleneglycol-mon-
obutylether at a concentration (0.2:0.3:0.5%) well above (>2
times) its critical micelle concentration in water. For comparison
extinction results for the MilSpec compliant RefAFFF formulation
and for two leading commercial fluorine-free AFFF formulations
(RF6, Solberg, Inc. product and Angus 3%, National Foam, Inc.
product) are also plotted. These results demonstrate the close
approach in pool fire extinction. Both foam spread and fire
extinction times are comparable to AFFF above 50 mL/min (1.75
kg/m.sup.2/min) application rate, as shown in FIG. 2 and FIG. 1
respectively. FIG. 1 shows that the commercial fluorine-free foams
contain excess solution for comparable fire extinction time. For
fixed liquid flow, extinction is faster with the siloxane
formulation than the commercial fluorine-free formulations. Because
only a limited amount of the solution can be carried to the fire
site, the commercial fluorine-free foams will not be able to put
out large fires as quickly as the siloxane formulation and AFFF on
per unit mass of liquid basis.
Six foot diameter pool fire tests outlined in MIL-F-24385F were
performed with a heptane pool. However, the fuel was changed from
gasoline to heptane in the present work. Only tests related to fire
extinction performance were performed in the current study. These
tests were conducted on candidate formulations prepared using fresh
water at full strength. The extinction time was measured from the
time of initiating deposition of the foam onto the 28 ft.sup.2
heptane pool fire, which had been burning 10 sec (pre-burn) before
starting the foam application, until the time of extinguishment.
The burnback test involved a reignition of the extinguished pool
fire after 90 sec of total foam application (includes time to
extinguish fire). The foam covered pool was reignited by lowering a
30.5-cm diameter pan of burning heptane-fuel into the center of the
pool and recording the time for fire re-involving 25% of the pool
surface. The film-and-seal test was conducted by covering the
cyclohexane fuel surface in a small container with foam, then
inserting a wire screen to scoop out the residual foam, waiting 60
sec then placing a small butane lighter flame approximately 12 inch
above the surface to ignite the fuel vapors permeating through the
water-surfactant film on the fuel surface. If the cyclohexane fuel
did not ignite, it received a pass.
It is important to note that the superior fire extinction
performance is partly due to a synergism between the
poly(oxyethylene)-trisiloxane and poly(glucoside)-alkane surfactant
components in that their use in combination far exceeds the
extinction performance of using equivalent quantities of each
surfactant alone. An example of this result is depicted by the plot
in FIG. 3. Data points along the x-axis represent no extinction in
FIG. 3. Such synergisms are not obvious or predictable and need to
be verified by experimental demonstration. Furthermore, a similar
synergism also exists in foam degradation as measured by the
lifetime of a foam layer placed on a hot fuel pool. As an example,
4-cm foam layer is applied onto n-heptane pool, which is maintained
at a constant temperature of 60.degree. C. as shown in FIG. 4. The
improved fuel/heat induced degradation of the combined surfactant
system contributes significantly to the superior fire suppression
performance of the siloxane formulation.
Another feature is that the length of the oxy-ethylene group can
significantly improve fire extinction. The oxy-ethylene group can
be on the trisiloxane surfactant, on the solvent and on the
hydrocarbon surfactant. Similarly, the size of glucoside group can
also improve the fire extinction. The numerical ranges of the m, n,
p, x, y, and z descriptors and the identity of R in the surfactant
structural formulae above, when combined in a
siloxane-glucoside-DGBE-water foam generating formulation, can
rapidly extinguish hydrocarbon fuel pool fires. Suppliers of
commercial surfactants in these two general categories will provide
the general formulae but the m, n, x, and y descriptors and R
identities are often considered proprietary. These surfactants
often have a dispersity of chain lengths making analyzed values of
the m, n, x, and y an averaged number. Evaluation of fire
suppression activity of foams generated from siloxane-glucoside
formulations containing these commercial surfactants finds some to
be highly effective. By using analytical monodisperse or
synthesized surfactants with known m, n, x, and y parameters and
known R identities, numerical thresholds and ranges were defined
for these parameters and used to calibrate m, n, x, y, and R of
commercial surfactants as well. An example using .sup.1H NMR
spectral measurements to calibrate the structural features of m and
y of the poly(oxyethylene)-trisiloxane surfactant is depicted in
FIG. 5.
FIGS. 6 to 10 show examples of variations in the structural
parameters of the two surfactants, solvent, and variation in the
composition and surfactant amount, and their effects on fire
extinction. Data points along the x-axis represent no fire
extinction in FIGS. 6 to 10. As examples, FIG. 6 shows fire
suppression performance of the commercial poly(oxyethylene)
trisiloxanes shown in FIG. 5 as formulations. As the number of
oxyethylene units (parameter m) increases, the fire suppression is
shown to increase, with 67A having low suppression and 502W having
high fire suppression. There is a range of oxyethylene chain
lengths for a formulation to be most effective as a fire
suppressant. As examples, FIG. 7 shows the effect of variation in
parameters n and x in the glucoside surfactant structure shown in
FIG. 1 on the fire extinction. Glucopon 225DK and Glucopon 215CS UP
have a mean length of tail (n=8 to 10) but different lengths of
head with x values of 0.7 and 0.5 respectively. Glucopon 600CS UP
has a mean length of the tail (n=12-14) and head length x=0.4.
Siloxane formulations were prepared with 502W/Glucopon/DGBE,
0.2/0.3/0.5%. Triton CG425 has longer alkyl chain length of 8 to
14. The effects of glucoside unit length, x, and alkyl chain
length, n, on fire suppression are significant. The solvent also
can affect the fire extinction. As examples, FIG. 8 shows the
effect of increasing the parameter z, which is the length of
oxyethylene chain, diethylene glycol butylether, and
triethylenglycol butylether. Siloxane formulations were prepared
with solvent/502W/Glucopon, 0.5/0.2/0.3%.
A methodology is disclosed to rank numerous (14) commercial
surfactants and numerous (14) siloxane formulations, and identify
the siloxane formulation described above. The chemical structures
of the commercial siloxane and hydrocarbon surfactants are shown in
FIGS. 11 and 12 and serve as comparative examples to 502W
commercial siloxane surfactant. The methodology consists of ranking
surfactant chemical structures by their fuel resistance properties,
which are measured in the absence of a fire. The fuel resistance
properties are correlated with fire extinction performance. An
example of fire extinction measurement consists of the foam
application on to a burning pool described above but using a 12-cm
diameter heptane pool, instead of the 19-cm diameter pool. Use of a
smaller pool size to evaluate lower performing surfactants and
formulations enables measurement of fire extinction times for
quantitative comparison and structure-property correlation among
surfactants. Fire extinction times are measured at different values
of the foam flow rates. For the purpose of establishing the
correlation, relative fire extinction was defined as the foam flow
rate needed to achieve 30 second fire extinction and is expressed
as relative to the measured value for RefAFFF formulation (140
mL/min for 30 s fire extinction). The fire performance of most
hydrocarbon surfactants and siloxane surfactants could not be
quantified by conducting fire extinction with the 19-cm diameter
heptane pool fire. The order of ranking obtained by the fire
performance agrees with the ranking by measured fuel resistance
properties in the absence of a fire as discussed below.
Fuel resistance properties include measurements of foam degradation
rate by fuel and fuel vapor diffusion rate through a foam layer
placed on top of a fuel pool, which should be maintained at a
constant temperature. An example of the apparatus, measurement
methods used, and results were described elsewhere ("Measuring Fuel
Transport through Fluorocarbon and Fluorine-free Firefighting
Foams", Fire Safety Journal, 91, 653-661 (2017) and "Influence of
Fuel on Foam Degradation for Fluorinated and Fluorine-free Foams",
Colloids and Surfaces A, 522, 1-17 (2017)). As an example, foams
were generated the same way as in the fire extinction measurements
described above by aspirating inert gas (nitrogen is used instead
of air to prevent potential fire) at a constant flow rate (900
mL/min). Foam flow was directed onto a hot heptane pool placed in
an open beaker to form a 4-cm thick foam layer quickly. The bottom
part containing fuel in the beaker was placed in a hot water bath
to maintain a constant fuel temperature. As shown in FIG. 13,
change in foam height was recorded with time to measure the
degradation induced by the exposure to hot fuel.
Similarly, as an example, measurement of fuel transport is
described below. To measure fuel transport rate through foam, fuel
and foam were introduced into the bottom half of a glass chamber in
the same way as in the foam degradation experiment. The bottom part
of the chamber was placed in hot water bath to maintain the fuel
temperature at 60.degree. C. The glass chamber was then closed
tight and nitrogen gas was continuously fed (500 mL/min) into the
chamber. The gas swept the surface of the foam carrying any fuel
vapors permeated through the foam into FTIR, which recorded fuel
vapor concentration with time until the foam degraded, exposing the
bare fuel pool (19.6 cm.sup.2 area). To obtain fuel vapor
suppression fraction versus time, the fuel concentration was
measured by the FTIR with the foam covering the pool divided by the
measured concentration (5675 ppm, 1.3.times.10.sup.-7
mole/cm.sup.2/s) for bare heptane fuel. The suppression fraction
with time is shown in FIG. 14. The fuel vapor concentration at the
foam surface was obtained by multiplying the suppression fraction
with the fuel vapor concentration on uncovered heptane pool (i.e.
vapor pressure of heptane at 60.degree. C., 29.5 vol %). FIG. 14
shows that the commercial fluorine-free foams (RF6 and Angus) have
fuel resistance inferior to the siloxane formulation.
Foams were generated using a commercial surfactant solution by
itself and as part of the formulation shown in Table 1 at a total
surfactant concentration 4 to 10 times the critical micelle
concentration. Time for complete degradation of 4-cm layer foam by
the fuel is indicative of foam degradation rate for a given
surfactant. Relative foam degradation rate is defined as the time
for complete degradation of 4-cm thick foam layer generated using
RefAFFF formulation divided by the corresponding value for the
candidate surfactant. Similarly, time taken for the fuel vapor
concentration at the foam surface to reach the lower flammability
limit for heptane (1 volume %) is indicative of the fuel transport
rate for a given surfactant. Relative transport rate is defined as
time to reach 1 volume % on the surface of the foam layer generated
from RefAFFF formulation divided by the corresponding value for a
candidate surfactant.
FIG. 15 shows a correlation between relative fuel transport rate
through foam and relative foam degradation rate for 28 commercial
siloxane and hydrocarbon surfactants and their formulations. FIG.
15 shows how the chemical structure variations shown in FIGS. 11
and 12 affect the fuel resistance properties of the foams. FIG. 15
shows that the fluorocarbon surfactant formulation RefAFFF near the
origin having the slowest fuel transport rate and foam degradation
rate (highest fuel resistance properties, time for complete
degradation of foam: 3800 s, time for fuel vapor concentration to
reach 1 vol. % at foam surface: 3619 s). All of the fluorine-free
surfactants are placed at different distances from the origin,
based on their relative rates (relative rate=time for RefAFFF/time
for a candidate foam). Among the surfactants tested, the most
commonly used sodium dodecyl sulfonate (SDS) and a siloxane
SilsurfJ208 (Siltec Inc.) are farthest from the origin indicative
of their low fuel resistance properties. FIG. 15 shows that the
siloxane formulation ("502WForm" is 502W/Glucopon215UP/DGBE
0.075/0.05/0.5% by volume) and the leading commercial fluorine-free
formulation (RF6, Solberg Inc.) are the closest to RefAFFF among
the surfactants tested. FIG. 16 shows a correlation of the relative
fire extinction with the relative fuel transport rate for 28
commercial siloxane and hydrocarbon surfactants and their
formulations. The fluorocarbon surfactants (Capstone1157, RefAFFF)
are closest to the origin and SDS and SilsurfJ208 are the farthest
indicating that faster fuel transport results in longer fire
extinction time. Again, the siloxane formulation (502WForm) and the
commercial fluorine-free formulation, RF6 are the closest to
RefAFFF (foam flow needed to achieve 30 s extinction time=140
mL/min, time for fuel vapor concentration to reach 1 vol. % at foam
surface=3619 s). The ranking of various surfactant by their
distance from origin generally follow the trends shown in FIG. 15;
few exceptions such as Capstone1157 with DGBE is due to the
synergistic effects caused by the solvent. Because the relative
extinction was defined based on foam flow rate rather than liquid
flow rate, the siloxane formulation (502WForm) appears closer to
RF6. On per unit liquid basis, the siloxane performs better than
RF6. Relative fire extinction was defined as the foam flow rate
needed to achieve 30 second fire extinction and is expressed as
relative to 140 mL/min.
A summary of extinction results corresponding to commercial
surfactants evaluated by themselves and as part of formulation
(where capstone is replaced by the fluorine-free surfactant in
RefAFFF and denoted as "Form") are shown in Table 2. 502WForm shown
in Table 2 consists of 502W/GlucoponCS215UP/DGBE of 0.075/0.05/0.5%
and has an extinction time of 25 s. Siloxane formulation shown in
Table 2 consists of 502W/Glucopon225DK/DGBE of 0.2/0.3/0.5% and has
one of the longest fuel transport and foam degradation times among
the fluorine-free formulations tested.
TABLE-US-00002 TABLE 2 Fire extinction time for foams generated by
different surfactant solutions for 12-cm diameter n-heptane pool
with 2-cm lip and 30 seconds preburn. Extinction is based on foam
flow rate, not liquid flow rate. Transport time to reach 1
Degradation Extinction @ vol % time for 500 mL/min on foam 4-cm
foam Surfactant foam flow surface, s layer, s RefAFFF 12 3619 3800
Capstone1157 + DGBE 12 2790 2700 RF6 17 478 1620 502WForm 25 448
840 Capstone1157 38 2710 2100 Glucopon215UP 40 433 190 TritonX100
44 138 270 Tergitol TMN6Form 70 182 150 502W 70 126 500 Tergitol
15-S-7Form No extinction 272 360 501WForm No extinction 198 195
AlfafoamerForm No extinction 190 195 Alfafoamer No extinction 190
150 Tergitol 15-S-7 No extinction 142 195 SilsurfForm No extinction
122 115 Tergitol TMN6 No extinction 94 90 SilphosJ208 No extinction
86 135 SilphosForm No extinction 78 250 SDS No extinction 67 67
SilsurfJ208 No extinction 57 45 501W No extinction 20 20 67A (0.5%)
NA 76 120 Glucopon215UP + NA 470 260 DGBE TritonX100 + DGBE NA 130
180 Angus 3% NA 158 780 Siloxane Formulation NA 630 1380
Dynamic surface tension is important for making high quality foams
with small bubble sizes. The dynamic surface tension as measured by
KRUSS bubble tensiometer and example results are shown in FIG. 17.
The siloxane formulation reaches low surface tension very quickly
compared to the commercial fluorine-free foams (RF6, Solberg Inc.
and Angus/National Inc.'s Respondol 3%). This is consistent with
small bubbles observed for the siloxane formulation compared to
other hydrocarbon surfactant based formulations shown in FIG. 17.
The rapid reduction in surface tension is important because bubbles
are formed rapidly in large scale fire application where
pressurized nozzle is used to generate the foam rapidly from
solution. Surprisingly, the siloxane foam reaches low values of
surface tension more rapidly than AFFF. However, AFFF is expected
to reach lower value of the surface tension at long times than the
siloxane foam. Therefore, aqueous film formation is not expected to
occur for the siloxane formulation unlike that of AFFF.
Table 3 shows solution properties of three siloxane formulations in
columns 3 to 5, two commercial fluorine-free formulations (RF6 and
Angus) in columns 6 and 7, commercial AFFF (Fomtec Inc.) in column
8, and RefAFFF in column 9. As expected, fluorine-free formulations
have near zero or negative spreading coefficients.
TABLE-US-00003 TABLE 3 Comparison of siloxane surfactant
formulations with a commercial AFFF formulation (Fomtec),
commercial fluorine-free formulations (3% concentrate Respondol
DS1617 ATF 3/3 of Angus Inc., 6% concentrate RF6 of Solberg/3M Co.
2005) and Mil Spec criteria. 2:3 502W/ 3:2 502W/ 1:3 502W/ MilSpec
Test Criteria Glucopon225DK Glucopon225DK Glucopn225DK Angus 3% RF6
FomtecAFFF RefAFFF 3% concentrate >2 5.3 6.29 5.09 5892 * 4.3
3.19 viscosity at 20.degree. C. (cP) 3% concentrate <20 7.41
8.23 6.9 NA NA 8.92 4.73 viscosity at 5.degree. C. (cP) Premix
solution NA 1.14 1.11 1.14 12 2.75 NA 1.12 viscosity (cP) 3%
concentrate >1.363 1.3706 1.3720 1.3709 1.3656 1.3701 1.3737
1.3617 refractive index 3% concentrate pH 7-8.5 6-8 6-8 6-8 6-8 6-8
8.22 6-8 Premix solution NA 22.4 21.95 22.9 23.2 26.25 NA 15.2
surface tension (mN/m) Premix solution NA 2.289 2.587 2.008 1.0
2.557 NA 4.483 interfacial tension with cyclohexane (mN/m) Premix
solution >3 -0.4 -0.2 -0.6 -3.4 -4.5 6.27 8.1 spreading
coefficient on cyclohexane (mN/m) * Too high to measure
Additional testing compares bench scale performance to large pool
performance. Fire extinction measurements were conducted for the
compositions shown in Table 4. Transport, degradation, and other
solution and foam properties were also measured. Table 4 shows the
compositions of three Siloxane-Gluc formulations and the RefAFFF
formulation used for making the foams. The percentages of
surfactants and DGBE refer to the amounts of the surfactant
concentrates and DGBE supplied by the respective manufacturers. The
surfactant concentrations shown in Table 4 for the Siloxane
formulations are two and half (3:2 Siloxane-Gluc215) to ten times
(2:3 Siloxane-Gluc225 and 2:3 Siloxane-Gluc600) the respective CMC
values, and the RefAFFF is 5 times the CMC value. Increasing the
concentrations of the siloxane and glucoside surfactants to 0.3%
and 0.2% respectively in 3:2 Siloxane-Gluc215 formulation shown in
Table 4 did not result in a significant change (<10%) in fire
extinction, degradation, and transport properties in the bench
scale measurements possibly because they are significantly higher
than CMC.
TABLE-US-00004 TABLE 4 Fluorine-free siloxane surfactant
formulations and fluorinated RefAFFF formulation. The values shown
under each column are volume percentages of the individual
components (or concentrates) in distilled water. The formulations
were used for foam generation, property and fire performance
measurements. 3:2 Cap- 2:3 Siloxane- 2:3 Siloxane- 3:2 Siloxane-
Gluc215 Gluc225 Gluc600 Gluc215 (RefAFFF) 0.2% 502W 0.2% 502W
0.075% 502W 0.3% Capstone 0.3% Glucopon 0.3% Glucopon 0.05%
Glucopon 0.2% 225 DK 600 CS UP 215 CS UP Glucopon 215 CS UP 0.5%
DGBE 0.5% DGBE 0.5% DGBE 0.5% DGBE
Fire extinction time measurements using the benchtop heptane
pool-fire apparatus was described previously to compare RefAFFF,
commercial AFFF, and commercial fluorine-free foams (Conroy et al.,
"Surface Cooling of a Pool fire by Aqueous Foams", Combustion
Science and Technology, 189, 806-840 (2017); Hinnant et al.,
Surfactant and Detergents, 21, 711-722, (2018); Williams,
"Properties and Performance of Model AFFF Formulations", Workshop
on Firefighting Foams in the Military, Naval Research Laboratory,
Washington, D.C., (Dec. 16-18, 2004)). Here, the fire suppression
data for a commercial AFFF (Buckeye 3%) and the four formulations
shown in Table 1 are compared, namely the RefAFFF, the
Siloxane-Gluc225, Siloxane-Gluc600, and Siloxane-Gluc215
surfactants formulations. In a 19-cm diameter heptane pool fire
using a foam application rate of 1000 mL/min, at 0 seconds, the
foam is introduced to the pool fire surface after the pool has been
burning for 60 seconds. Within the first 5 seconds of foam
application, a significant suppression is not observed in all
cases. After 10 seconds of foam application, the 3:2 Cap-Gluc215
(RefAFFF) formulation extinguished most of the fire (knockdown)
similar to a commercial AFFF (Buckeye), while the Siloxane-Gluc225
formulation did not suppress the fire to the same degree After 15
seconds there was complete extinction by Buckeye and RefAFFF, while
Siloxane-Gluc225 suppressed most of the fire (knockdown).
Siloxane-Gluc225 took longer (20 seconds) to completely extinguish
the fire unlike the other two fluorinated foams, 3:2 Cap-Gluc215
and Buckeye 3%, which took 12 and 16 seconds respectively for
complete extinction. For the two fluorinated foams and the
fluorine-free foam, fire persisted for a few seconds above the foam
even in the regions of the pool covered with the foam and also
subsequent to complete coverage of the pool by the foams. In the
case of the two fluorinated foams, the fire persisted above the
foam layer for as long as 50% of the extinction time and may
underscore the significant role the foam layer plays in fire
extinction relative to any "aqueous film" layers that may exist
underneath the foams. Also, the persistent fire above the foam
layer may be indicative that the fuel vapor emanating from the hot
pool surface permeates through the foam layer feeding the fire
above. The fuel transport through the foam ceases as the foam layer
thickens due to continued application of the foam, resulting in
fire extinction due to lack of the fuel supply. During the
extinction process, foam also degrades and delays building a thick
foam layer. This can be noticed at very slow foam application
rates, where the foam was unable to cover the pool despite
continuous application of the foam for a long time (up to 6 min)
because foam was degraded by the hot fuel and the fire. At high
flow foam application rates subsequent to the fire extinction, the
residual foam layer disappeared quickly with time especially for
the fluorine-free foams. Thus, high fuel transport and high foam
degradation can increase the minimum volume of foam (or minimum
foam layer thickness) needed to extinguish a fire, which is a
performance measurement of a given formulation (For example,
MilSpec requires a 28 ft.sup.2 fire to be put out in 30 s using
less than 1 gallon of surfactant solution, which translates to 5 to
10 gallons of foam depending on the expansion ratio). It is
difficult to measure fuel transport and foam degradation during the
rapid extinction process. However, they can be measured under
controlled conditions as performance characteristics of a given
formulation.
FIG. 18 shows extinction times measured for bench top (19-cm
diameter) and large scale (6-ft diameter) heptane pool fires as
functions of foam application rate per unit area (flux) of the
pool. The 6-ft pool fire test is same as the MilSpec MIL-F-24365F
but the gasoline fuel is replaced with heptane. For the benchtop,
fire extinction times for the Siloxane-Gluc225 surfactants
formulation (solid square) are compared with RefAFFF (solid circle)
and the commercial AFFF (Buckeye 3%, solid diamond) foams. As the
foam application rate is decreased, the extinction time increases.
and When the extinction time is greater than 180 seconds, the foam
application is stopped and the fire is extinguished by placing a
tray over the pool. The RefAFFF and the Siloxane-Gluc225
formulations could not extinguish the flame within 180 seconds at
foam application rate below 5.9 and 9.7 L/m.sup.2/min respectively.
The extinction times for the siloxane formulation are closer
(<1.5 times that of RefAFFF) to the AFFFs at large foam
application rates. For the 6-ft (1.8 m) heptane pool fire, the
extinction times for the Siloxane-Gluc225 formulation are compared
at fixed solution flow rates of 7.6 and 11.4 L/min (2 and 3 gallons
per minute) which correspond to 18.6 and 22.2 L/m.sup.2/min of foam
flow rates respectively. The foam flow rates are calculated by
multiplying the measured liquid flow rates with the measured
expansion ratio values. The extinction times for the
Siloxane-Gluc225 formulation are compared with that of RefAFFF
formulation for the 6-ft heptane fire in FIG. 18. The extinction
times are 45 and 30 seconds for the Siloxane-Gluc225 and RefAFFF
respectively at a fixed foam flux of about 22 L/m.sup.2/min. Thus
the extinction times for the siloxane formulation are within 1.5
times those for the RefAFFF, consistent with the bench-scale data
for the same foam flux. Despite significant differences in the foam
generation and foam properties between the bench and large scales,
the fire extinction data are surprisingly consistent possibly
because the surfactant-fuel interactions and the foam application
rate per unit area have more significant effects. Although the foam
application rate 22 L/m.sup.2/min is about the same for the two
formulations, the solution application rate 11.4 L/min (3
gallon/min, expansion ratio 5.1) is higher for the Siloxane-Gluc225
than 7.6 L/min (2 gallon/min, expansion ratio 7.5) for the RefAFFF
because of the differences in the foam expansion ratio in the large
scale testing. The foam expansion ratio of Siloxane-Gluc225
decreases from 6.4 to 5.1 as the solution flow rate increases from
2 gpm to 3 gpm during the large scale foam generation. The
extinction data shown in FIG. 18 are plotted as function of
solution application rate in FIG. 19. Comparing FIG. 18 with FIG.
19, the large and small scale data are closer for a fixed foam
application rate rather than for a fixed liquid application rate as
one may expect. Also, for a fixed extinction time of 51 seconds,
the foam application rate is 1.5 times higher for the large scale
heptane pool than for the small scale data shown in FIG. 19.
The fluorinated surfactant formulation (RefAFFF) was able to
extinguish the heptane pool fire in 90 seconds as the foam
application rate was decreased to less than 5.9 L/m.sup.2/min in
FIG. 18 (solid circles). Replacing the fluorocarbon surfactant with
a commercial siloxane surfactant in a simple four-component
formulation required only 50% greater foam application rate (9.7
L/m.sup.2/min) to achieve an equivalent extinction time (90
seconds) as shown in FIG. 18 (solid squares). Given the simplicity
of the formulations evaluated compared to a commercial formulation,
the fire suppression performance of the Siloxane-Gluc225 is
reasonably good, and may lead to further improvements in fire
suppression with further optimization.
Fire suppression was conducted in the 6-ft diameter pool (28
ft.sup.2) MilSpec standard pool fire by foams generated from
Siloxane-Gluc225 and Cap-Gluc215 (RefAFFF) formulations listed in
Table 5 using heptane as the fuel so that the results can be
compared with the bench-scale results. Tests were performed at
solution flow rates of 2 gpm and 3 gpm (expansion ratio 5.1) and
with Cap-Gluc215 (RefAFFF, expansion ratio 7.5) at 2 gpm. Even
though the solution application rates are different between
Siloxane-Gluc225 and RefAFFF formulations, the foam application
rates (22 L/m.sup.2/min or 57.3 L/min or 15 gpm) shown in 3.sup.rd
row of Table 5 are about the same because of the higher expansion
ratio measured for RefAFFF (expansion ratio 7.5) than for
Siloxane-Gluc225 (expansion ratio 5.1); the foam application rates
are calculated by multiplying the solution flow rates with the
expansion ratio and are not measured directly as a part of the
MilSpec testing. The foams are applied at 0 seconds after the
heptane pool has burned for 10 seconds. After 15 seconds, the
Siloxane-Gluc225 did not suppress the fire to the extent RefAFFF
did. After 30 seconds, the Siloxane-Gluc225 suppressed most of the
fire while RefAFFF completely extinguished it. The fire extinction
time for the siloxane-Gluc225 decreased from 51 to 45 seconds as
the solution flow rate increased from 2 to 3 gallon/min compared to
the extinction time of 30 seconds for the RefAFFF.
TABLE-US-00005 TABLE 5 Comparison of Siloxane-Gluc225 formulation
at 2 and 3 gpm and with RefAFFF at 2 gpm liquid application rate
for 6-ft diameter Mil Spec MIL-F-24385 pool fire using heptane as
the fuel instead of gasoline.sup.1. Criteria RefAFFF (based Mil
Spec test 3:2 Cap/ on with heptane fuel 2:3 Siloxane-Gluc225
Gluc215 gasoline) Liquid flow rate (gpm) 2 3 2 2 Foam flow
rate.sup.2 (gpm) 12.8 15.3 15 N/A 90% extinction (s) 43 26 21 N/A
Extinction (s) 51 45 30 <30 Burnback (s) 338 424 981 >360
Film and seal N/A N/A PASS PASS Expansion ratio 6.4 5.1 7.5 5-10
25% Liquid Drainage (s) 198 198 251 >150 Average bubble size
(.mu.m) 220 .+-. 111 140 .+-. 30 170 .+-. 30 N/A 3% concentrate
viscosity, 5.3 5.3 3.2 >2 20.degree. C. (cP) 3% concentrate
viscosity, 7.4 7.4 4.7 <20 5.degree. C. (cP) Solution viscosity,
20.degree. C. 1.14 1.14 1.12 N/A (cP) Spreading coefficient on -0.4
-0.4 6.4 >3 cyclohexane.sup.3, (mN/m) at 20.degree. C.
Interfacial tension on 2.2 2.2 1.9 N/A cyclohexane, (mN/m) at
20.degree. C. Surface tension (mN/m) at 22.4 22.4 16.7 N/A
20.degree. C. 3% concentrate refractive 1.371 1.371 1.362 >1.363
index CMC (% volume of total 0.05 0.05 0.1 N/A surfactant
concentrates) 3% concentrate pH 6-8 6-8 6-8 7-8.5 .sup.1The MilSpec
results for unleaded, alcohol-free, gasoline fire suppression using
RefAFFF were given in Hinnant et al., Surfactants and Detergents,
21, 711-722, (2018) .sup.2Foam flow rate is the specified liquid
flow rate multiplied by the measured foam expansion ratio
.sup.3Surface tension of cyclohexane is 25 mN/m at 20.degree.
C.
A subset of five metrics in the MilSpec standard MIL-F-24385 were
focused on to evaluate the Siloxane-Gluc225 and the RefAFFF
formulations. Five parameters were measured and compared with
passing criteria, which are based on gasoline fuel rather than the
heptane. The parameters measured were (1) 28 ft.sup.2 gasoline pool
fire extinction time, (2) burnback time, (3) film and seal, (4)
expansion ratio, and (5) 25% drainage time, and are described in
MilSpec standard MIL-F-24385. The results are shown in Table 5.
Both 90% and 100% extinction times decrease as the foam flow rate
is increased as shown in Table 5. At a fixed foam flow rate of
about 15 gallons per minute shown in row 3 and columns 3 and 4 of
Table 5, the 90 and 100% extinction times for the Siloxane-Gluc225
are less than or equal to a factor of 1.5 times those for RefAFFF.
However, at a fixed solution flow rate of 2 gallons per minute, the
extinction times differ by as much as a factor of two as shown in
columns 2 and 4. The factor 1.5 times is consistent with the
bench-scale data as shown in FIGS. 18 and 19. One reason for the
longer extinction time for Siloxane-Gluc225 is the larger foam
degradation rate, as indicated by the smaller burnback time of 338
s for the siloxane formulation compared to 981 s for the RefAFFF as
shown in columns 2 and 4. Foam is applied for a total of 90 s
including extinction, therefore the expected burnback time for the
siloxane formulation is 544 s, after correcting for the shorter
foam application time following fire extinction and for the
difference in the expansion ratios. The measured burnback time of
338 s for the siloxane-Gluc225 is significantly smaller than the
expected 544 s indicating significantly greater foam degradation.
The data in Table 5 show that the increased foam flow rate
decreases the extinction time. The foam flow rate can be increased
by increasing the foamability or the expansion ratio as well as by
increasing the solution flow rate. The expansion ratio and liquid
drainage time are smaller for the siloxane formulation than for the
RefAFFF. However, increasing the expansion ratio can decrease foam
spread rate on the pool and prolong fire extinction. Therefore, in
addition to reducing the foam degradation, varying composition of
the foam solution to optimize the expansion ratio may also
significantly reduce the extinction time. Other properties,
especially the viscosity of the concentrate, are within MilSpec
criteria. This is important because many commercial fluorine free
concentrates have viscosities well above the MilSpec criteria.
The reason for relatively good fire suppression performance of the
Siloxane-Gluc225 formulation is the synergism between the Siloxane
and Glucoside surfactants indicated by the smallest foam flow rate
at which the formulation can extinguish the fire in a given time
(e.g., 180 seconds). FIG. 20 depicts the synergism where the
surfactant mixture can extinguish the fire at a much smaller foam
application rate than the individual surfactant solutions rather
than an intermediate foam flow rate, which is expected following
the law of averages. Data points along the x-axis represent no fire
extinction. FIG. 20 shows the bench scale heptane pool fire
extinction data for 3:2 Siloxane-Gluc215 formulation composition
listed in Table 4 and for the three individual surfactant solutions
(0.45% Glucopon 215 CS UP, 0.1% siloxane 502W, 0.5% siloxane 502W,
0.5% Glucopon 225 DK, 0.5% Glucopon 600 CS UP all with 0.5% DGBE in
distilled water). Gluc215 could not extinguish the fire even at a
very high foam flow rate (2100 mL/min or 74 L/m.sup.2/min within
180 s) as shown by the data points (open squares) on the x-axis,
which represent no extinction. Similarly, 502W siloxane surfactant
solution also could not extinguish the fire below a high value of
the foam flow rate (1550 mL/min or 54.7 L/m.sup.2/min) as
represented by the data points (solid circles) on the x-axis. But,
when both the surfactants are combined in a 3:2 ratio the foam
extinguished the fire at significantly smaller foam flow rate (in
197 s at 453 mL/min or 16 L/m.sup.2/min) as indicated by the data
(solid squares) for Siloxane-Gluc215 (composition 0.075% 502W,
0.05% Gluc215, and 0.5% DGBE) exhibiting synergism. Similar results
are shown for Siloxane-Gluc225DK and Siloxane-Glucopon600UP
mixtures.
FIG. 20 also shows the effect of varying Glucopon surfactant's head
size and the number of OH functional groups on heptane pool fire
extinction because of the synergistic effects. The head size is
varied from x=0.4, 0.5, and 0.7 using commercially available
Glucopon 600 CS UP, Glucopon 215 CS UP, and Glucopon 225 DK
respectively while keeping the composition fixed (0.2% 502W, 0.3%
Glucopon, and 0.5% DGBE). Gluc600 has slightly longer alkyl tail
than Gluc215 and Gluc225. Increasing the hydrophilicity of the
hydrocarbon surfactant increases the synergistic effect, and
reduces the foam flow rate where the extinction time is 180 s in
FIG. 20. The synergistic extinction between 502W and Glucopon
surfactants is unique because for most other commercial surfactants
that were examined, the extinction times fell between the
extinction times of the individual surfactant following the law of
averages. The synergism is responsible for the high extinction
performance of the siloxane formulation.
The synergistic extinction between 502W siloxane and Glucopons
relates to the synergistic foam degradation, which is shown in FIG.
21. Again, FIG. 21 shows that the surfactant mixture exhibits
smaller foam degradation rate than the rates for the individual
surfactants rather than an intermediate value, which is expected
following the law of averages. FIG. 21 shows percent change in foam
layer thickness (initial thickness of 4-cm) on top of a hot heptane
pool with time due to fuel vapor induced degradation. The foams
generated from 0.5% Glucopon 215 CS UP with 0.5% DGBE and 0.1% 502W
with 0.5% DGBE degraded in 240 and 480 s respectively. The foam
generated from Siloxane-Gluc215 (0.075% 502W, 0.05% Glucopon 215 CS
UP, and 0.5% DGBE) degraded completely in a much longer time (900
s) compared to the degradation times of the individual surfactants.
FIG. 21 also shows degradation data for the individual surfactant
solutions of Gluc600 and Gluc225 and their mixtures with 502W. They
also show synergistic effects like those described for
Siloxane-Gluc215. Furthermore, as the glucoside head size of the
Glucopon is increased, the degradation rate is reduced for the
glucoside mixtures with the siloxane surfactant. The synergism is
increased by increasing the hydrophilicity of Glucopon's head group
as shown by the foam degradation for Siloxane-Gluc600,
Siloxane-Gluc215, and Siloxane-Gluc225 formulations in FIG. 21. The
number of OH functional groups increased from x=0.4, 0.5 to 0.7 by
switching from Gluc600, Gluc215 to Gluc225; Gluc600 has slightly
longer alkyl tail. The exact mechanisms for the increased foam
stability are not well understood. However, it is possible that the
increased hydrophilic interactions between the polyoxyethylene and
glucoside head groups may have reduced the surfactant solubility in
heptane resulting in increased foam stability over the individual
surfactants. FIG. 21 shows that the foam generated from
Siloxane-Gluc225 formulation degraded completely in 1380 s versus
900 s for the foam generated from Siloxane-Gluc215 (in a 3:2
ratio), and 500 s for Siloxane-Gluc600. Siloxane-Gluc225 has the
smallest degradation rate but is still significantly higher than
the RefAFFF as shown in FIG. 21. For comparison, commercial AFFF
(Buckeye 3%) and RefAFFF degrade completely in time periods that
are 1.7 (2400 s) and 2.6 times (3600 s) longer respectively than
the time (1400 s) for Siloxane-Gluc225.
FIG. 22 shows that the fuel permeation rate through foam follows
roughly the law of averages and does not show the synergistic
effects observed in foam degradation at small times for all three
Siloxane-Gluc surfactant combinations. For example, below 450 s,
the fuel flux for foam generated from Siloxane-Gluc215 lies between
those for foams generated from the individual surfactants. However,
the synergism exhibited in foam degradation in FIG. 21 prolongs the
foam life time and suppresses the fuel flux shown in FIG. 22 at
long times. At longer than 450 s, the surfactant mixture
Siloxane-Gluc215 has a smaller fuel flux than the individual
surfactants, which exhibit a steep rise in fuel flux because of
differences in the foam degradation rate. Thus, the synergism in
degradation leads to a dramatic reduction in fuel transport rate
for the three surfactant mixtures compared to foams generated with
individual surfactants because of the increased lifetimes of the
foams for the mixtures. Even though the fuel flux is smaller for
Gluc225 than for Siloxane-Gluc225 mixture at short times,
Siloxane-Gluc225 lasts longer (1750 s) than Gluc225 (980 s) and the
trend reverses. For comparison, commercial AFFF (Buckeye 3%) and
RefAFFF have a heptane flux of 0.4 (2.7.times.10.sup.-9
mol/cm.sup.2/s) and 0.1 (7.times.10.sup.-11 mol/cm.sup.2s) times
that of Siloxane-Gluc225 (6.6.times.10.sup.-9 mol/cm.sup.2/s)
respectively at 1000 s. The reduction in degradation and fuel
transport rates decreases the fire extinction times for the
mixtures over the individual surfactants shown in FIG. 20. The
foams last longer in the fuel transport apparatus because it is
closed and the water vapor is contained unlike in the open
degradation apparatus. This is especially true for Gluc225 which
lasts greater than 980 s in the transport apparatus but less than
400 s in the degradation apparatus. But FIG. 22 shows a steep rise
in fuel flux to 1.times.10.sup.-8 mol/cm.sup.2/s in less than 200 s
for the Siloxane foam. It takes much longer than 200 s for the foam
to degrade completely and for the fuel flux to reach that of bare
heptane's fuel flux (not shown in FIG. 22). The reason the Siloxane
foam's fuel transport curve rises rapidly in FIG. 22 is that the
heptane vapors travel through the foam very quickly and not because
of a significant reduction in foam layer thickness. Thus, the
Siloxane foam seems to have significantly higher fuel transport
than Gluc225 foam and the Gluc225 foam has only a slightly higher
degradation rate than the Siloxane foam. However, the combination
of Siloxane with Gluc225 suppresses the foam degradation
dramatically and as a result the fuel transport is also suppressed
at long times. The fuel transport rate depends on the fuel
concentration in surfactant solution, which may depend on the
micelle size and number density because most of the fuel is
expected to reside inside the micelles. Micelle size can affect the
diffusion rate. The exact mechanisms of fuel and micelle transport
in the presence of a surfactant remain unclear.
If a foam spreads too slowly, it can increase the extinction time
because complete coverage of the pool surface is necessary, but not
sufficient, to extinguishing a fire. FIG. 23 shows the time to
cover a burning heptane pool with foam, which is delivered at the
center of the bench-scale pool at a constant flow rate for the
Siloxane-Gluc225 and the RefAFFF formulations listed in Table 4. As
the foam flow decreases the foam spread time increases as expected.
Also shown are the fire extinction times for comparison. The foam
spread times are about half the extinction times at high foam flow
rates for both the formulations. As the foam flow rate is
decreased, the foam spread times become greater than half, but
still remain smaller than the fire extinction times. This is likely
because foam degrades significantly due to longer exposure to the
hot pool and fire. In addition to foam degradation, foam spread
also depends on the rheological properties and the expansion ratio,
which increases as the foam flow rate decreases in our foam
generation apparatus as discussed later in the paper.
Table 6 shows the surface and interfacial tension values for the
individual components and mixtures of the Siloxane formulation and
the RefAFFF formulation listed in Table 4. The surface tension and
interfacial tension values for the Siloxane-Gluc225DK are close to
those of the 502W component and the Gluc225DK component
respectively. It may support indirectly that 502W may adsorb
preferentially on air-water interface while Gluc225DK may adsorb on
the heptane-water interface similar to that suggested for
fluorocarbon and hydrocarbon surfactants in the literature (Kissa,
"Fluorinated surfactants and repellants", Surfactant Science
Series, 97, New York, Marcel Dekker Inc. (2001)). The surface
tension and interfacial tension measurements do not exhibit
synergistic effects for the Siloxane-Gluc225DK formulation because
the mixture values fall in between those for the two components.
Therefore, the surface and interfacial tensions and spreading
coefficient values do not explain the synergistic effects shown in
fire extinction time data for Siloxane-Gluc225DK in FIGS. 20-22.
Even though, the spreading coefficient values listed in Table 6
show that only RefAFFF but not Siloxane-Gluc225DK can form an
aqueous film on heptane pool surface, the spreading coefficient
does not consistently explain fire extinction performance
differences among the individual components and mixtures unlike
fuel transport and foam degradation measurements shown in FIGS.
20-22. Even for AFFF, differences in spreading coefficient did not
explain differences in fuel transport through a foam layer covering
a pool when different fuels were used (Hinnant et al. "Measuring
Fuel Transport through Fluorocarbon and Fluorine-free Firefighting
Foams", Fire Safety Journal, 91, 653-661 (2017)).
TABLE-US-00006 TABLE 6 Surface tensions and interfacial tensions
for surfactant formulations and individual components with 0.5%
DGBE at 20.degree. C. Interfacial tensions and spreading
coefficients were measured with heptane (surface tension, 20 mN/m).
They are different from those in Table 4 measured with cyclohexane.
Surface Interfacial Spreading Formulation Tension Tension
coefficient 0.1% 502W 21.1 5.6 -6.7 0.5% DGBE 0.5% Gluc225DK 28.1
2.9 -11.0 0.5% DGBE Siloxane-Gluc225 22.4 2.9 -5.3 Ref AFFF 16.7
1.9 1.4
Ability to achieve a low value of the surface tension quickly is
traditionally been considered a requirement for effective fire
suppression. The dynamic and equilibrium surface tensions of the
siloxane formulation were compared with AFFF and with individual
surfactant components and no synergistic effects were found.
Dynamic surface tension can play a role in foam generation and
affect foam properties. During foam generation, surfactant should
be able to diffuse from the solution quickly and adsorb on freshly
created bubble surfaces. As more surfactant is adsorbed, the
surface tension decreases with time and reaches a steady state when
the bubble surfaces are saturated. FIG. 24 shows that the surface
tension of Siloxane-Gluc225 reaches steady state value within a few
seconds like AFFFs. But, the initial decrease in surface tension is
much quicker for the fluorine-free formulation than for the
fluorinated formulations (AFFFs) at short time scales (<1 ms).
The siloxane formulation is able to adsorb and decrease the surface
tension of a freshly created bubble surface quicker than AFFFs.
This is an important property for the large scale foam generation
where the bubbles are generated at a high frequency. However, due
to the lack of a fluorocarbon surfactant, the steady state value of
the surface tension is higher for the Siloxane-Gluc225 formulation
than for the AFFFs. Furthermore, the individual surfactants have a
surface tension-time profile similar to the mixture and no
synergism in reducing the surface tension of water is exhibited
unlike the observations reported for AFFFs. It was reported that
the fluorocarbon and hydrocarbon surfactants exhibited a synergism
where the dynamic surface tension of the mixture decreased quicker
than the individual surfactants (Dlugogorski et al., "Dynamic
Surface and Interfacial Tensions of AFFF and Fluorine-free Class B
Foam Solutions", Fire Safety Science-Proceedings of Eighth
International Symposium, International Association for Fire Safety
Science, 719-730 (2005)). Despite the Siloxane-Gluc225
formulation's ability to achieve steady state surface tension
quicker than the fluorinated formulation (RefAFFF), the fire
extinction times for the fluorine-free foams are consistently
higher than for the RefAFFF. Therefore, the differences in dynamic
surface tension are not responsible for the observed differences in
fire extinction, fuel transport, and foam degradation shown in
FIGS. 20-22.
FIG. 25 shows the static or steady state surface tension at
different concentrations of the total surfactant, which is the sum
of 502W and Glucopon concentrates or the sum of Capstone.TM. 1157
and Glucopon 215 CS UP concentrates supplied by the manufacturers.
Below the CMC, as the concentration of surfactant in solution is
increased, the concentration adsorbed onto the air-solution
interface increases, and the surface tension decreases as shown in
FIG. 25. Above the CMC, the interface is saturated with adsorbed
surfactant and the steady-state surface tension value depends on
the formulation. In FIG. 25, the CMC value is determined as the
concentration where the surface tension becomes independent of the
total surfactant concentration. The concentrations of the total
surfactants listed in Table 4 are eight times greater than CMC
value of 0.06% for the Siloxane-Gluc225, two times greater than the
CMC value of 0.06% for Siloxane-Gluc215, and five time greater than
the CMC value of 0.1% for RefAFFF. For comparison, individual
surfactant solutions of Gluc225, Gluc215, Gluc600, 502W, and
Cap1157 containing equal amount DGBE solvent have CMC values of
0.1, 0.1, 0.06, 0.06, and 0.06% of the surfactant respectively. The
CMC values for the Siloxane-Gluc mixtures are closer to the
Siloxane surfactant's CMC. The CMC value for the RefAFFF is closer
to Gluc215. FIG. 25 shows that the surface tension-concentration
profiles and the CMC values are very similar between the
fluorine-free and fluorinated formulations.
Ability of a foam to extinguish a fire may depend on foam's bubble
structure and foam properties. So initial bubble size
distributions, liquid drainage profiles, and initial expansion
ratio of Siloxane foam were compared with those of AFFF. As soon as
foam is generated, the liquid begins to drain from the foam, the
bubbles begin to coarsen, and average bubble size increases. The
initial bubble size distribution depends on the composition of the
surfactant formulation for a given generation method. FIGS. 26 and
27 show the "initial" bubble size distributions for foams generated
using different methods at large and bench scales respectively.
FIG. 26 shows the distributions for Siloxane-Gluc225 generated at 2
and 3 gpm solution flow rates through an air-aspirated nozzle, and
for RefAFFF generated at 2 gpm within 2 minutes after foam (30.5-cm
high foam column) is collected into a rectangular glass cylinder
(4.2 cm.times.4.2 cm.times.30.5 cm). The measurements are made by
taking pictures with a digital camera placed at 13-cm below the
surface of the foam column. As the solution flow rate is increased
from 2 to 3 gpm, the bubble distribution for the Siloxane-Gluc225
formulation seem to approach that of the RefAFFF. This may be
because the RefAFFF and Siloxane-Gluc225 generated at 2 gpm and 3
gpm respectively have the same foam flow rate of 15 gpm (see Table
5). The average (arithmetic mean) bubble size decreases from 220
.mu.m to 170 .mu.m as the solution flow is increased from 2 to 3
gpm and approaches 140 .mu.m for the RefAFFF. About 50 to 100
bubbles are divided into six bins to create each distribution curve
shown in FIG. 26. FIG. 27 shows that the bubble size distributions
for the air sparged foams of Siloxane-Gluc225 are close to those
for the RefAFFF at bench-scale within 30 seconds after the foam
(25-cm high, 4 cm diameter foam column) is collected into a glass
cylinder of the DFA analyzer. The measurements and analysis are
made by the DFA's camera and the associated software at 13-cm below
the top surface of the foam column. About 3900 to 4900 bubbles are
divided into 50 bins to create each distribution curve in FIG. 27.
The percentage of bubbles shown on the y-axis are smaller compared
to those in FIG. 26 because six to eight times more bins are used
to create the distributions. FIG. 27 shows that the bench-scale
generated bubble sizes are similar to those generated at the large
scale and the distributions depend on the surfactant solution
application rate. The RefAFFF and Siloxane-Gluc225 foams have
similar bubble size distributions at a fixed foam application rate
but show significant differences in fire extinction. FIG. 27 also
shows bubble size distributions for the individual surfactants and
there may be slightly less number of smaller bubbles compared to
the surfactant mixtures, but the differences in average bubble
diameters are not significant at 30 seconds after foams are
generated.
FIGS. 28 and 29 show increase in average bubble size with time due
to coarsening that occurs when air from small bubbles diffuses to
the large bubbles driven by the differences in air solubility
(Oswald-Ripening). The differences in air solubility in the
surfactant solution are caused by the differences in bubble
curvature. FIG. 28 shows that coarsening occurs more slowly in the
RefAFFF than in the siloxane formulation after the first few
minutes at the large scale. It is unclear to what extent the
differences in coarsening between the fluorinated and fluorine-free
foams shown in FIG. 28 affect their fire extinction behavior that
occurs at time scale of 30 seconds. FIG. 29 shows the coarsening
behavior in fluorinated and fluorine-free foams generated at the
bench scales are almost the same unlike at the large-scale shown in
FIG. 28. Despite the similar coarsening behavior, there are
significant differences in fire extinction times between the
fluorinated and fluorine-free foams at bench-scale. FIG. 29 also
shows the bubble coarsening for the individual surfactants. Within
the time scale of extinction (<180 seconds), the differences in
average bubble sizes between the individual and mixtures of
surfactants is less than 50%. The slight degree of synergistic
effects on average bubble sizes appear to be not significant enough
to explain the large synergistic effects on foam stability and fire
extinction shown in FIGS. 20-22.
Liquid drainage is a characteristic of the foam that depends on
foam generation method and the associated bubble size
distributions. The liquid drains because of competition between
gravity and capillary forces that depend on the bubble size
distributions in the foam. FIGS. 30 and 31 show the rate of liquid
drainage from foams generated from Siloxane-Gluc225 and RefAFFF
formulations at large and bench scales. At large scale and
bench-scales, the liquid drainage profiles were measured by
collecting the foams in to graduated cylinder (5 cm diameter and 28
cm height) and into the DFA cylinder (4 cm diameter and 25 cm
height) respectively. The initial expansion ratio measured
immediately after foam generation increases as the liquid drains
out of the foam and can be calculated using the curves in FIGS. 30
and 31. The drainage rate is significantly faster in the air
sparged foams at bench scale than in the air aspirated nozzle foams
at large scale. However, the drainage rates are similar for
fluorinated and fluorine-free formulations at bench-scale and large
scale. A similar drainage characteristic does not seem to imply
similar fire extinction times because the two formulations have
different extinction times as shown in FIGS. 18 and 19.
The initial expansion ratio of the foam delivered onto the fuel
pool can also depend on foam flow rate and generation method. The
initial expansion ratios measured at two flow rates are shown in
Table 5 for the large scale MilSpec aspirated nozzle. The aspirated
nozzle generates wetter foams than the sparging method used in the
bench-scale experiments for the siloxane and RefAFFF formulations.
The initial expansion ratios of the foams generated at the
bench-scale using the sparging method are shown in FIG. 32. The
trends are very similar among different formulations studied in the
bench-scale experiments. In the sparging method, foam collects on
top of the solution above the sparger disk. At high flow rates, the
residence time of the foam collected in the cup is small and liquid
drainage during that time is negligible. As the flow rate is
decreased, the foam residence time increases and liquid drainage
increases. The expansion ratio increases with decreasing flow rate
as shown in FIG. 32. At 1000 mL/min used for foam characterization
shown in FIG. 26-31, the expansion ratios vary slightly between 7.8
to 9.5 for the fluorinated and fluorine-free formulations. The
small differences in expansion ratio do not explain the differences
in foam stability and fire extinction seen in FIGS. 20-22.
The siloxane formulations using fluorine-free surfactants can be
used to generate foams with fuel vapor resistance property and fire
suppression activity that exceeds that of leading commercial
fluorine-free formulations and approach the fire extinction
performance level of fluorocarbon surfactant containing AFFF
formulations having MilSpec qualification. The fluorine-free
feature is critical for environmental regulation compliance. It
also enables the selection of already commercialized siloxane and
glucoside surfactants to produce a formulation with a fire
suppression capability approaching that required by MilSpec and
currently only fulfilled by fluorocarbon surfactant containing AFFF
formulations. A methodology was developed where the fuel resistance
property measurements were used as metrics to quantitatively rank
numerous commercial formulations that enable identification of
superior performing fluorine-free surfactant relative to AFFF. By
carefully choosing a systematic variation in the chemical
structures of the surfactants, this methodology is capable of
providing structure-property relationships quantitatively.
Fluorocarbon surfactants differ significantly from the flurine-free
surfactants in their hydrophobic and oleophobic interactions with
water and fuel leading to superior foam properties and fire
performance. However, combining two fluorine-free surfactant
structures in a mixture can exhibit synergistic effects leading to
superior performance over the individual components. A commercial
Siloxane surfactant with a polyoxyethylene head group when combined
with a certain commercial alkane surfactant with polyglucoside head
group exhibited quicker fire extinction of a heptane pool fire than
the individual components. This was due to synergistic reduction in
foam degradation rate (caused by the heptane vapors generated by
the hot fuel pool) for the mixed surfactant formulation over the
individual surfactants. Whether the large siloxane tail and slender
polyoxyethylene head of the siloxane surfactant and the inverse for
the alkylglucoside sufactant enables bilayer formation is unclear.
The molecular intercations between the two surfactants' hydrophilic
head groups (poly oxyethylene and poly glucoside) and precise
mechanisms of the synergism are unclear. However, increasing the
number of --OH functional groups by increasing the size of the
polyglucoside head, reduced the foam degradation and the fire
extinction time further. Indeed, small differences in the size
(x=0.4, 0.5, and 0.7) of polyglucoside head had a significant
effect on foam stability. It is possible that the stronger
interaction between the two head groups may have suppressed the
surfactant solubility in heptane resulting in increased foam
stability near the foam-fuel interface. Previously, it has been
shown that the fuel destabilizes the foam near the foam-fuel
interface causing coalescence of bubbles in a cascading effect
leading to rapid degradation (Hinnant et al., "Influence of Fuel on
Foam Degradation for Fluorinated and wo Fluorine-free Foams",
Colloids and Surfaces A, 522, 1-17 (2017)). Similarly, the micelle
size and number density can affect the fuel concentration and
diffusivity, which may affect the fuel transport. The mechanisms of
transport for the individual versus combined surfactants are also
unclear. The synergism between siloxane and glucoside structures
resulted in a factor of 5 enhancement in foam stability and fire
extinction performance demonstrating the key role played by the
interactions between the surfactant structures in extinction.
Developing understanding of molecular interactions between the
fluorine-free surfactants at an interface and in micelles, and an
approach based on synthesizing synergistic molecules can result in
performance matching that of fluorocarbon surfactants.
The fire extinction time for the siloxanesurfactant formulation to
be less than 1.5 times that of an equivalent AFFF formulation
containing a fluorocarbon surfactant for both the bench (19-cm
diameter) and 6-ft diameter heptane pool-fires at a fixed foam
application rate (22 L/m.sup.2/min). Previous works (U.S. Pat. Nos.
9,446,272 and 9,687,686) relied on aqueous film formation and less
volatile fuel (deisel) rather than the foam dynamics and
synergistic effects to enhance fire suppression on a volatile fuel
(heptane). Furthermore, the viscosity of the siloxane-formulation
concentrate is within the MilSpec criteria unlike many commercial
fluorine-free firefighting-foam concentrates (Solberg Inc.,
https://www.solbergfoam.com; Angus Inc.,
http://angusfire.co.uk/products, Fomtec Inc.,
https://www.fomtec.com, Chemguard Inc).
The difference in fire extinction between the Siloxane-Glucoside
and RefAFFF formulations was due to differences in foam degradation
and fuel vapor transport rates rather than the differences in
surface tension (dynamic and static) or aqueous film formation,
bubble size distributions and coarsening, foam spread rates, and
liquid drainage rates for the foam application rates studied.
Synergistic effects in foam properties are unclear and single
lamella studies are needed to directly relate surfactant effects to
a bubble lamella stability. Solution and foam properties cannot be
ignored because they may become the controlling factors for fire
extinction depending on the specific surfactant system under
consideration and the foam generation methods used.
Siloxanes are known to undergo hydrolysis in water during long term
storage. FIG. 33 shows an accelerated aging test where 3%
concentrate was kept for 10 days at 65.degree. C. in an oven. It
showed no loss in 28 ft.sup.2 pool fire suppression capacity.
Obviously, many modifications and variations are possible in light
of the above teachings. It is therefore to be understood that the
claimed subject matter may be practiced otherwise than as
specifically described. Any reference to claim elements in the
singular, e.g., using the articles "a", "an", "the", or "said" is
not construed as limiting the element to the singular.
* * * * *
References